unable to make or receive calls on a Emporia 5

[JaneRed_2159307]

My husband uses Smarty and his Emporis 5 phone which supports 4g has suddenly stopped receiving and making calls. I have done all the recommended checks to no effect. he still receives msg and emails etc. I contacted the Phone company who say check if SIM is ok 

[SmartyTrousers]

My guess is that his phone is not compatible with Smarty's 4G calling (VoLTE), even though it may be compatible with the 4G calling on some other networks. Previously the phone will have dropped down to 3G to make or receive calls, but Three (Smarty's parent network) have now turned off 3G across most of the UK.